Image scanning devices disclosed in patent documents 1 through 4 include light sources that are disposed away from a document surface and do not include reflectors. Such a configuration makes it possible to reduce production costs, to scan high-quality images by reducing flare, and to reduce the size of an image scanning device.
Patent document 1 discloses an image scanning device in which flare is reduced by placing a light source away from a document surface and removing a reflector. However, the image scanning device disclosed in patent document 1 has the following problems (1) through (4).
Patent document 2 discloses an image scanning device in which production costs are reduced by eliminating a reflector. However, similar to the above, the image scanning device disclosed in patent document 2 has the following problems (1) through (4).
Patent document 3 discloses an image scanning device including an image sensor and light sources arranged above and below the image sensor. The light sources share an imaging optical system of the image sensor. Light emitted from the light sources is illuminated onto a scanning position on the document surface, with the use of a mirror arranged near the document surface. However, the image scanning device disclosed in patent document 3 has the following problems (1) through (4).
Patent document 4 discloses an image scanning device in which an image sensor and a light source share an imaging optical system to scan and illuminate the same position on a document surface with the use of a half mirror. However, the image scanning device disclosed in patent document 4 has the following problem (5).
The above-mentioned problems (1) through (5) are described below.
(1) The imaging optical axis and the illumination optical axis are substantially the same and perpendicular with respect to the document surface. Therefore, the reading value changes according to the luster gloss of the document. (At least, the angles of these light axes with respect to the normal line of the document surface are not specified).
(2) The light source is arranged away from the document surface. Therefore, in order to illuminate a sufficient amount of light on the document surface, it is necessary to provide an intensive light source or a condenser such as a lens, which leads to increased costs. Furthermore, an increased amount of power is required for driving the light source.
(3) If a light source is arranged beneath the imaging lens, the height of the image scanning device will be increased. This cancels out the effect of making the image scanning device compact by eliminating the reflector and the like.
(4) The illumination optical axis does not completely coincide with the imaging optical axis. This causes a shortage in the amount of illumination for scanning a lifted-up portion of the document or a portion at the center of a book document.
In particular, in the image scanning devices disclosed in patent documents 1 and 2, first and second carriages travel while scanning a two-dimensional document image. The positions of mirrors of the first and second carriages change intermittently according to the operation of scanning the document. Accordingly, the positions at which illuminated light reaches the document surface change. As a result, the brightness of the scanned image changes, and high-grade images cannot be scanned.
(5) By providing a half mirror, the cost of the image scanning device will increase. Furthermore, each time the illumination light emitted from the light source is reflected by the half mirror and when the light reflected from the document surface passes through the half mirror, the light amount decreases by one-half. As a result, the final valid illumination amount becomes a quarter of that emitted from the light source, thus increasing the power loss of the power source. Moreover, illumination light irradiated from the half mirror toward the lens is inevitably reflected from the lens surface and is then incident on the image sensor. This constantly causes flare. Consequently, it is difficult to achieve a sufficient SN ratio in image scanning.
As described above, in the image scanning devices disclosed in patent documents 1 through 4, the utilization rate of the light quantity emitted from the light source is extremely low.
Patent document 5 discloses an illuminating device for a projector. The illuminating device disclosed in patent document 5 is basically used for irradiating a surface. Hence, patent document 5 does not disclose a method for efficiently irradiating a line. Furthermore, patent document 5 does not disclose a method of applying the illuminating device to an image scanning device.
Examples of a typical image scanning device are described with reference to FIGS. 1 through 4.
FIG. 1A is a schematic diagram of a typical image scanning device and FIG. 1B is a cut-away side of the image scanning device in the sub-scanning direction.
In an image scanning device 100, a document 107 is placed on a contact glass 108. Light emitted from a lamp 109 and light reflected from a reflector 110 that received the light emitted from the lamp 109 are irradiated onto an imaging area 111 of the document 107. The light reflected from the document 107 is reflected by a deflecting mirror 113 in a first moving body 103, and also by a turn-around mirror A 112a and a turn-around mirror B 112b in a second moving body 104, and is imaged onto a one-dimensional image sensor 101 by an imaging lens 102. In this manner, the one-dimensional image sensor 101 acquires a one-dimensional image of the linear imaging area 111. The direction in which the one-dimensional image sensor 101 acquires this one-dimensional image is referred to as a main scanning direction.
In the image scanning device 100, the first moving body 103 and the second moving body 104 receive a driving force from a motor 105 through a drive transmitting unit 106. The first moving body 103 travels at a speed that is twice as high as that of the second moving body 104. As a result, the imaging position of the imaging lens 102 with respect to the surface of the contact glass 108 is maintained on the surface of the one-dimensional image sensor 101, while light travels along the surface of the contact glass 108 in a direction perpendicular to the linear imaging area 111 and parallel to the contact glass 108. In this manner, the one-dimensional image sensor 101 sequentially scans an image of the document 107 placed on the contact glass 108, so that a two-dimensional image is acquired. The direction in which the first moving body 103 and the second moving body 104 travel is referred to as a sub-scanning direction.
Generally, a one-dimensional CCD is used as the image sensor. The imaging lens 102 reduces the image on the surface of the contact glass 108, and focuses the reduced image on the one-dimensional image sensor 101.
The traveling speed ratio of the first moving body 103 and the second moving body 104 is set at 2:1, i.e., the movement distance of the second moving body 104 is half of that of the first moving body 103. The distance from the imaging area 111 to the imaging lens 102 or to the one-dimensional image sensor 101 is constant, regardless of the positions of the first moving body 103 and the second moving body 104.
Generally, the image resolution of a scanner is expressed in DPI (dots per inch). The image resolution of a scanner installed in a digital PPC often falls in a range of 400 DPI through 600 DPI. A color scanner employs three CCDs, each having a sensitivity for a light spectrum of R (red), G (green), and B (blue). The optical path lengths between each of the CCDs and the document are equal. A 3 line CCD may be employed as an image sensor, which 3 line CCD is used for R (red), G, (green), and B (blue) and is arranged in a sub scanning direction. In this case, the distance between the pixel rows is approximately 4 dots through 8 dots of the main scanning reading area of CCD pixels. Each pixel row is not necessarily integrated. Accordingly, if the 3 line CCD is used as the image sensor in the above-described image scanning device, the scanning positions on a document corresponding to CCD pixels of R, G, and B, will be different. Therefore, light for illuminating the document needs to be irradiated at scanning positions corresponding to each of the colors.
FIG. 2 is a schematic diagram of another type of image scanning device.
In this type of image scanning device shown in FIG. 2, the reducing optical system includes an image sensor 201 and an imaging lens 202. A document 204 is placed on a platen 203, and an image of the document 204 is scanned, without providing an optical system such as mirrors between the image sensor 201 or the imaging lens 202 and the document 204. The present invention can be applied to this type of image scanning device. In the image scanning device shown in FIG. 2, when a one-dimensional CCD is employed as the image sensor 201, a two-dimensional image of the document 204 can be scanned by scanning the document 204 on the platen 203 along one direction or by causing a unit including the imaging lens 202 and the image sensor 201 to travel along one direction. Generally, in such an image scanning device, natural light (room light) is used as the illumination light or a light source that uniformly illuminates the platen 203 is provided. However, if natural light is used, the light quantity of natural light will be unstable, and the illuminance of natural light on the document 204 will often become low. Therefore, this image scanning device may not be able to scan images of the document 204 with high quality.
Next, problems (1) through (5) of a typical image scanning device are described.
(1) Energy Saving in Image Scanning Device
In a scanner acting as an image scanning device, the illumination lamp is the element that consumes the largest amount of power. Particularly, as the image scanning speed increases, the charge storage time of the CCD will decrease. As a result, an illumination lamp with high brilliance will be needed and power consumption of the scanner will increase.
Conventionally, the light source provided in a scanner has changed from a halogen lamp to a xenon arc lamp, and then to an LED, in order to achieve higher luminous efficacy.
FIG. 3 illustrates the relationship between an illuminating area and a scanning area in an image scanning device. FIG. 3 illustrates the relationship between an illuminating area 305 and a scanning area 302 of a scanner 300 acting as an image scanning device. As shown in FIG. 3, under the current conditions, the illuminating area 305 that is illuminated by illumination light 304 from a light source 303 is much larger than the scanning area 302 on the surface of a document 301 scanned by a CCD. For example, in a scanner of 600 dpi, the width of illumination necessary for scanning an image of the document 301 is 42.3 μm, whereas the actual width of the illumination area is approximately 20 mm. The simple energy efficiency of light obtained by comparing these widths is only about 0.5%. This means that energy is wasted for approximately 99.5% of the remaining light.
(2) Reducing the Size (Making a Thin-model) of Image Scanning Device
Efforts have been made to reduce the size of a scanner acting as an image scanning device by reducing the thickness of the scanner. In a reducing optical system of a scanner provided with a first moving body and a second moving body, a lamp and a reflector included in the first moving body are particularly the most problematic factors, which are limiting the layout of the first moving body. These elements have been obstacles in the attempt to make thinner scanners. Particularly, in the case of a digital PPC (plain paper copier) including a scanner, if the built-in printer is large, the position of the scanner surface for placing a document will become high. This will make it troublesome for a short person to place the document on the scanner.
(3) Reducing Costs of Image Scanning Device
A CCD and an imaging lens are the most costly elements in a scanner acting as an image scanning device. The next costly elements are an illumination lamp and accessories thereof. Particularly, a xenon arc lamp requires high voltage, and thus requires a power pack. Moreover, a lamp is provided in a first moving body, and therefore, a flexible power source line is necessary.
(4) Flare
A scanner acting as an image scanning device has a built-in illuminating device. In a typical scanner, a linear image of a document is scanned, and then a moving body moves along so that a linear image can be scanned at another position. This operation is repeated so that a two-dimensional image of the document can be scanned. (This type of scanning operation is referred to as line sequential image scanning. Particularly, scanning in the direction of reading a line is referred to as main scanning, and scanning in a direction perpendicular to that of the main scanning and parallel to the document surface is referred to as sub-scanning). Flare may occur in the course of scanning an image.
FIG. 4 is a diagram for describing illumination in an image scanning device and how flare occurs in the image scanning device.
In an image scanning device 400 employing a fluorescent tube as a light source 401, an illumination light 402 from the light source (fluorescent tube) 401 is irradiated onto a document surface 403, directly or via a reflector 404. The light irradiated on the document surface 403 is reflected from the document surface 403, passes through an opening 406 of the light source 401, and reaches a fluorescent surface 407 of the light source 401. Next, the light that has reached the fluorescent surface 407 is reflected from the fluorescent surface 407, and becomes a re-illumination light 405 illuminating the document surface 403 once again, thereby generating flare. (The illumination light that reaches the document surface from the illuminating device is referred to as a primary illumination light. The light that is reflected from the document surface and illuminates the document surface once again is referred to as a secondary illumination light).
If such flare occurs, even if an image area having a uniform density in a document is scanned, the image signals scanned by the scanner will vary due to different densities in the document around the scanning area. The image signals vary because when the primary illumination light is reflected from the document surface, the reflection light quantity changes according to different densities in the image of the document. As a result, the illumination light quantity, which is the total of the primary illumination light and the secondary illumination light, changes according to the document densities. Particularly, flare occurs considerably at portions where there is a rapid difference in the densities of the image in a document.
FIG. 5 illustrates an image scanned with an image scanning device, in which flare has occurred. In FIG. 5, a boundary portion 502 between black patterns 501 is scanned as a darker area than a white pattern 503. The white pattern in the image of the original document has a uniform density, and therefore, it is apparent that the quality of the scanned image of the boundary portion 502 is low (ideally, the boundary portion 502 and the white pattern 503 should have the same brightness). The boundary portion 502 is between the black patterns 501, and both edges outside the boundary portion 502 are black. Therefore, when the scanner scans the boundary portion 502, the secondary illumination light decreases relatively compared to that of the white pattern.
Generally, a scanner scans an area of a document with a low reflectance as a dark image, and scans an area of a document with a high reflectance as a bright image. That is, when the scanner scans a document with images of black characters, the white portions of the characters become relatively dark in the scanned image. As a result, contrasts in the image decrease, and may thus make it difficult to read the characters. This occurs because the secondary illumination light basically illuminates the document surface once again around the position from which the illumination light has been reflected. Accordingly, at portions where there is a rapid difference in the density of the image (e.g., a boundary portion between black and white patterns), the light quantity of the secondary illumination light varies significantly.
Thus, at the stage of designing the scanner, optical elements are coated black and the layout of the optical elements are adjusted, so that the secondary illumination light reflected from the document surface does not illuminate the document once again. However, it is not possible to completely prevent the secondary illumination light from being illuminated once again, and therefore, flare has been a problem in improving the quality of scanned images. Particularly, if portions around characters are distinctly darker than the characters, the background of a copied image will become soiled, thus creating an image with extremely low quality.
(5) Shadow in Book Document
FIG. 6 is a diagram for describing a shadow that appears in an image scanned from a book document. When scanning an image of a book document 601 placed on a contact glass 602 as shown in FIG. 6, a center portion 603 of the book document 601 is lifted up from the surface of the contact glass 602. When an image scanning device provided with a typical illumination optical system scans the book document 601 placed in this manner, an illumination light 605 does not reach a scanning position 604 corresponding to pixels of an image sensor. As a result, the scanned image becomes dark in this portion.
Accordingly, it is necessary to appropriately design the configuration and the arrangement of an illumination system in an image scanning device, in order to increase the utilization rate of light, save energy, make the image scanning device thin and compact, reduce the cost of the image scanning device, realize high image quality by reducing flare and preventing a shadow from appearing in an image of a book document, and reduce illuminance irregularities on the target surface.
Patent document 1: Japanese Laid-Open Patent Application No. 2000-253213
Patent document 2: Japanese Laid-Open Patent Application No. 2000-250146
Patent document 3: Japanese Laid-Open Patent Application No. H10-190990
Patent document 4: Japanese Laid-Open Patent Application No. H9-51405
Patent document 5: Japanese Laid-Open Patent Application No. 2003-280094